1. Field of the Invention
The present invention is generally related to lithography systems, and more particularly, to reticle positioning devices in a lithography system.
2. Background Art
Lithography is a process used to create features on a surface of a substrate. The substrate can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A semiconductor wafer, for example, can be used as a substrate to fabricate an integrated circuit.
During lithography, a reticle is used to transfer a desired pattern onto a substrate. The reticle can be formed of a material transparent to a lithographic wavelength being used, for example glass in the case of visible light. The reticle can also be formed to reflect a lithographic wavelength being used, for example extreme ultraviolet (EUV) light. The reticle has an image printed on it. The size of the reticle is chosen for the specific system in which it is used. A reticle six inches by six inches and one-quarter inch thick can be used, for example. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer corresponding to the image printed on the reticle.
The projected image produces changes in the characteristics of a layer, for example a resist layer, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conducting, semiconducting, or insulating layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
As should be clear from the above discussion, the accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer. The rigors of sub-100 nm lithography place stringent demands not only on the lithography tool, but also on the reticle. Airborne particles and dust that settle on the reticle can cause defects on the wafer. Small image distortions or displacements in the reticle plane can swamp critical dimension and overlay error budgets. A conventional solution is to use a thin piece of permanently fixed transparent material as a pellicle for the reticle.
This pellicle remains in place during all stages of the lithography process. A pellicle has a dual role in improving the accuracy of the image projected onto a wafer. First, a pellicle serves to protect the reticle from direct contact with particulate contamination. As discussed above, particles that settle on the reticle can produce image distortion, so they must be removed. However, removal of particles from the reticle can cause damage to the reticle because such removal may involve direct contact with the reticle. When a pellicle is used, particles will settle on the pellicle rather than the reticle. Thus, it is the pellicle that must be cleaned. Cleaning the pellicle rather than the reticle poses fewer dangers to the integrity of the reticle since the reticle is protected during this cleaning by the pellicle itself.
The second role played by a pellicle is related to the standoff of the pellicle. During exposure, the focal plane corresponds to the location of the image printed on the reticle. By including a pellicle, any particles in the system will settle on the pellicle rather than the reticle. By virtue of the thickness of the pellicle, and thus the distance between the surface of the pellicle and the patterned surface of the reticle, these particles will not be in the focal plane. Since the pellicle lifts the particles out of the focal plane, the probability that the image projected onto the substrate will include these particles is greatly reduced.
The pellicle solution works well in many conventional lithographic processing techniques. Since materials are available for producing transparent pellicles and reticles, the use of such a system is convenient in, for example, a system in which light must pass through both the reticle and the pellicle. The pellicle approach, however, is not well suited for use in extreme-ultraviolet (EUV) applications because there are no materials sufficiently transparent to EUV that can be used to make a pellicle. In EUV Lithography, light does not pass through the reticle, but is reflected off the object side of the reticle in a technique known as reflective lithography. If a pellicle were to be used in a reflective process, the EUV would necessarily pass through the pellicle twice; once on the way onto the reticle and again after reflecting off of the reticle. Thus, any amount of light absorbed from the pellicle would be effectively doubled if a pellicle were used in EUV processing techniques. In order to conserve enough light to perform photo-exposure with any measure of efficiency, EUV lithography preferably uses reticles without pellicles.
In the absence of a pellicle, contaminants will land on the surface of a mask and degrade the quality of the image reflected onto the wafers produced in EUV applications. Many techniques have been employed to reduce the number of contaminants that can land on a mask, in particular; extreme care is taken to ensure a clean environment around the mask, as well as to minimize the settling of any debris onto the mask area. For example, in addition to clean room environments and operating within a vacuum, such reticles are typically transported mask side down.
Nonetheless, particulate contaminants exist that could adhere to a mask; whether they are airborne in a clean environment or scattering through a vacuum. In particular, contact spots on a reticle used to hold it and transport it within the lithographic system can generate sub-microscopic fracture particles that can land on the mask as contaminants or debris. One type of contact spot is a region on a reticle that is physically touched by robotic devices, end effectors, grippers, or the like, to move and position the reticle. What is needed is an intermediary device that will interface with any robotic end effectors, grippers, or the like, in place of the reticle, while holding and securing the reticle with minimal contact to it.
In addition to the need for cleanliness within the lithography system is the need to transport them in a clean environment. For example, one trade association, Semiconductor Equipment and Materials International (SEMI) (see www.semi.org) has adopted standards for a Standard Mechanical Interface (SMIF) for pods used to transport and store both wafers and reticles. SEMI standard E100-0302 “Specification for a Reticle SMIF Pod (RSP) Used to Transport and Store Six Inch or 230 mm Reticles” is one such standard. This specification requires that when a reticle carrier is closed, the reticle must be centered with respect to the SMIF and must be secured in all degrees of freedom within the carrier to prevent movement during transport. Typically this is accomplished in the industry by clamping all sides and faces of the reticle between many opposing knobs of a shock-absorbing material in the SMIF pod. In the case of reticles in EUV lithography, there are no pellicles to protect reticle mask areas against particles resulting from RSP contact. The inventor has determined that there is a need for a device that holds the reticle with minimal contact and can interface with a SMIF pod, in place of the reticle, to assist in centering and securing the reticle.
In addition to the need for mask cleanliness, the need for accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer. The rigors of sub-100 nanometer lithography place stringent demands not only on the lithography systems performance in positioning the wafer, but also on its positioning of the reticle pattern. Reticle manufacturers place masks and patterns on their reticles within a known degree of accuracy with respect to that reticle's substrate. Trade associations such as SEMI have adopted common standards for definition of all physical characteristics of a reticle substrate, including position reference surfaces. For example, SEMI standard P37-1101“Specification for EUV Lithography Mask Substrates” is one such standard that defines all surfaces and dimensions with respect to a few small surface areas on that reticle. In order for a lithography system to establish the position of the reticle pattern accurately and precisely, it must first identify and establish the position of the substrate. The establishment of substrate position is typically a mechanical process known as registration.
In most lithography devices, the reticle is registered using a multiple-degree-of-freedom actuator device to grip and move the reticle until key features can be detected and confirmed by a multiple-degree-of-freedom sensor device. This process is contact-intensive, insufficiently accurate to achieve sub-100 nm alignment accuracy, and can take long enough to impact the productivity of the lithography system. For those reasons it is typically performed after the reticle is removed from the SMIF pod and before it is aligned to the wafer within sub-100 nm alignment accuracy. This process can be easily performed within most lithography devices because they are typically purged with clean gas and the reticles have protective pellicles. This process becomes extremely difficult in the case of an EUV lithography system, because of the lack of pellicles and the need to maintain a vacuum environment.
Another SEMI standard, SEMI E111-0303, defines the structure shown in FIGS. 1a and 1b. FIG. 1a is a top view of a reticle SMIF pod (RSP) 100 with a reticle 101. RSP 100 has four reticle contact surfaces 102 for positioning reticle 101 (only two of the contact surfaces are shown for clarity). FIG. 1b shows the reticle contact surface 102 in more detail within an enlargement area 104 corresponding to area 104 in FIG. 1a. SEMI E112-0303 discloses a similar SMIF pod structure designed to carry multiple reticles.
Correcting positioning of the reticle outside a SMIF pod with existing gripping devices causes the reticle to shift position significantly and to endure high surface stresses with every contact made. Furthermore, the reticle has to be gripped and released with every manipulation, in addition to those manipulations required to insert the reticle onto, or remove the reticle from, the device(s) within the lithography system that positions it within sub-100 nm accuracy. The inventor has found that this additional positioning effort dramatically increases the likelihood of sub-microscopic particle contamination. Thus, there is a need for an improved apparatus and method for holding and registering a reticle, within a vacuum environment, with minimal number of contacts made to that reticle.